A scumsided setup is one in which the anti-Town players (typically the Mafia) have the largest chance to win. If a game is very scumsided, this is normally considered a balance problem with the game (which would disqualify it from being Normal). Note that some styles of setup (e.g. setups with few power roles) have a natural tendency to be scumsided, on the basis that most attempts to improve the town's victory chances would do so via adding a random chance of a Town blowout (which leads to a rather arbitrary-feeling game), rather than via improving the average case. As such, setups that are only mildly scumsided are often tolerated, although it's nonetheless considered desirable to keep the Town and Mafia victory chances as close to equal as possible unless doing so would cause larger problems.
Along similar lines, a scumsided mechanic is one which increases the chance that the Mafia will win, and thus requires the setup to be designed in a way that's more favourable to the Town than it otherwise would be.
The opposite, a townsided setup, is one in which the Town has the largest chance to win. Just as with scumsidedness, this is typically tolerated in mild amounts and considered a balance problem if the setup is very townsided. In a setup with the usual Day/Night cycle and a standard nightkill, excess amounts of townsidedness are normally obvious when looking at the setup; it's hard for such a setup to be subtly townsided, because the town will often fail to believe that all the power role claims are genuine and end up lynching some of their most useful players. Nightless setups, however, tend to naturally fall on the townsided end of the scale; in particular, towns in this sort of setup tend to win much more often than the setup's expected value might suggest.